1. Field of the Invention
The present invention relates to a method for operating a vertical heat-treatment apparatus and more particularly to a method of transferring boats, each of which carries a lot of wafers, between a wafer transfer position and a boat load/unload position.
2. Description of the Related Art
As unmanned control system and automation system have recently developed in a clean room, there has been visualized a system for transferring wafer carriers (cassettes) by means of pilotless transferring robots between machines for manufacturing semiconductor devices. For this purpose, a loading/unloading port for automatically transferring wafer carriers is provided in each manufacturing machine.
It is required that the space of the clean room be as small as possible in order to exhaust duct in the clean room in a short time so as to maintain a high degree of cleanness. Since a vertical heat-treatment furnace has a smaller floor space than a horizontal heat-treatment furnace, the employment of the vertical heat-treatment furnace allows for effective usage of the floor space. Further, the vertical heat-treatment furnace has an advantage that a wafer boat is transferred without contacting the inner wall of a reaction tube.
In the vertical heat-treatment furnace is provided a wafer transfer device for transferring semiconductor wafers from the carriers to the boat.
U.S. Pat. No. 4,770,590 discloses a wafer transfer mechanism for a vertical heat-treatment furnace. However, this wafer transfer mechanism is of a semiautomatic type so that an operator must manually set carriers on the station. At the time of this setting, undesirable dust enters the furnace and semiconductor wafers are contaminated, as a result increasing the fraction defective of the semiconductor devices.